It is hard to see how anything good will come from President Reagan's announcement that the United States will no longer be limited by the SALT II nuclear arms treaty with the Soviet Union. It takes the cap off the arms race at the moment when Congress and the American public show signs of wearying of its pace. As far as relations with our allies are concerned, it is a real loser. And it is likely to be a political loser for the Republicans here at home as well.

In a damage-control operation, the administration sent Secretary of State George Shultz and Secretary of Defense Caspar Weinberger out to do "Meet the Press" and "Face the Nation" on Sunday. They made reasoned cases for the decision. The treaty signed by Jimmy Carter and Leonid Brezhnev in 1979 had never been ratified, they said, and some of its terms had been violated already by the Russians. If the Russians demonstrate their sincerity by this autumn, the United States may yet agree to observe the limits.

But when all was said and done, viewers knew that a president who in more than five years has not come close to negotiating any new arms-control agreement had unilaterally decided to ignore numerical limits on nuclear missiles that the Russians and the United States had accepted and observed for the past seven years.

Canada's Conservative foreign minister Joe Clark was shown saying on television, ''This is a profoundly disturbing development which we had hoped would not occur.'' And reports from last week's NATO meeting in Halifax were that the United States had no support among the 15 allied nations for this decision.

European countries that two years ago accepted a new generation of medium-range nuclear missiles on their territory, in return for Reagan's pledge to seek arms control, were vehement. Even Britain's Margaret Thatcher, the president's best buddy, took a walk on this one.

The attempted cover-up of the Chernobyl nuclear power plant accident a few weeks ago had reminded Europe and the rest of the world of the Soviets' callousness. Now Reagan has taken them off the hook and handed them an issue they well know how to exploit.

Predictably, the Russians said that if the lid is off for the United States, they will accelerate their side of the nuclear arms race as well. The higher level of competition makes sense only if you believe that the United States will shovel in dollars faster than the Russians do rubles. Weinberger left no doubt Sunday that he believes we can -- and should.

But Weinberger has not been persuasive to the majority of members of Congress of either party for at least two years. The budget resolutions passed by the House and Senate this year allow the Pentagon barely enough of an increase to cover inflation. Two weeks ago, 46 senators signed a letter saying that spending for the Strategic Defense Initiative (Star Wars) should rise only 3 percent next year -- not the 74 percent Reagan had asked.

In the face of the budget deficit, the only way the president and Weinberger can blackjack Congress into raising the ante for new military hardware is to whip up a big war scare. And that is extremely dangerous to the health of the Republican Party and its chances of keeping control of the Senate this year and of the White House in 1988.

A Gallup Poll released last week (but taken in early March) showed Republicans enjoyed the biggest lead over the Democrats as the party of prosperity in the 35-year history of the survey. By a 51-33 margin those polled said they thought Republicans were better bets to keep the nation's economy healthy.

But the same poll showed the parties at a virtual standoff (39 percent Republican; 36 percent Democratic) on keeping the peace. For all his popularity, Reagan never has convinced a majority of the voters that he is a good bet for avoiding war.

In that respect, he is unlike other Republican presidents in modern times. In 1952 and 1956 under Dwight D. Eisenhower's banner, the Gallup Poll showed Republicans had gained the party of peace label by wide margins. They did so again under Richard M. Nixon in 1968 and held it even in 1972, when the Vietnam war continued and George McGovern ran as a Democratic "peace candidate."

Today's serious doubts about the Republicans' handling of the war-and-peace issues are particularly striking because the Democrats have not mounted any broad attack on Reagan's foreign policy in the past 18 months. While opposing specific actions, such as the arms sale to Saudi Arabia and assistance to the contras in Nicaragua, opposition spokesmen have not pushed their demands for a "nuclear freeze" or other approaches to arms-control as Democratic presidential contenders did in the two years before the last election.

Ever since arms talks resumed in Geneva early in 1985, Reagan has had a free ride at home on his handling of nuclear issues and the Soviet-U.S. relationship. That domestic truce is sure to break down over his SALT II decision. Reagan has handed the Democrats an issue when they really needed one.